This invention relates generally to electrical resistance heaters and, more particularly, to electrical sheath heaters.
Sheath heaters, such as cartridge heaters, conventionally comprise a resistance element, typically coiled on an insulating core, and a metal sheath that is coaxial with the coil and core and radially spaced from the coil. A mineral insulating material having an optimum combination of relatively high thermal conductivity and relatively low electrical conductivity fills the space between the sheath and the coil. Power is delivered to the heater through internal lead pins contained within longitudinal holes in the core where they are held in electrical contact with the resistance element. External leads, having insulating sleeves, are connected to the lead pins and extend through an open end of the heater.
When power leads are in high temperature environments (e.g., in excess of 1000.degree. F.), insulating sleeves often become baked out and brittle. When this happens, any amount of flexing of the external leads causes the sleeves to fray and thereby expose the conductors, with the likely result being electrical shorting between the conductors or a grounded surface.
Another problem frequently encountered by sheath heaters is contamination of the internal components. Contamination occurs when foreign materials enter the heater and cause a breakdown of the coil or surrounding insulation. The contaminants may cause a gross electrical shorting or an accelerated deterioration of the internal elements of the heater. Heaters that are subject to large temperature swings or frequent cycling are most susceptible to ingesting harmful contaminants. When a heater heats up and cools down causing thermal expansion and contraction of the air inside it, it "breathes" surrounding gasses or liquids--as well as any contaminants within the gasses or liquids.
Another problem encountered by sheath heaters is contamination by the heaters of the environments they are used in. Contamination (i.e., outgassing) occurs when moisture and/or gasses escape from the heater. In some applications--such as when the heater is being used in a vacuum environment--outgassing may be undesirable or unacceptable.